Thin-film hard disc magnetic media are widely used for on-line data storage in computers. There are several magnetic and surface properties which are important to the recording density, as well as to disc lifetime and wear characteristics. The most important of these are:
(1) Magnetic remanence, which determines the signal amplitude which can be read from an isolated pulse stored in the medium--the greater the remanence, the greater the signal amplitude which can be detected in a reading operation.
(2) Coercivity, defined as the magnetic field required to reduce the remanence magnetic flux to 0, i.e., the field required to erase a stored bit of information. Higher coercivity in a medium allows adjacent recorded bits to be placed more closely together without mutual cancellation. Thus, higher coercivity is associated with higher information storage density.
(3) Bit shift or peak shift, a phenomenon which refers to the broadening between voltage peaks which occurs in the read voltage waveform It is desired to achieve low bit shifting, inasmuch as bit shifting limits the resolution at which adjacent peaks can be read, and thus places an upper limit on recording density.
(4) Flying height, i.e., the distance which a read/write head floats above the spinning disc. Less overlap of voltage signals in adjacent magnetic domains in the disc occurs as the read/write head is moved closer to the disc surface, allowing recording density to be increased. The flying height is limited principally by surface irregularities in the disc.
(5) Surface imperfections and irregularities. Such can lead to excessive interactions between the disc and the head, and limit the flying height of the head which can be safely employed.
(6) Stiction, or static friction, defined as the frictional contact between the disc and read/write head when the head is parked on the disc and the disc first begins to rotate. Low stiction reduces wear on the disc and head with repeated stop/start operations.
(7) Surface durability, which provides increased disc lifetime and reduced head wear.
Heretofore, discs having high coercivity and remanence characteristics have been prepared by sputtering a thin magnetic film on a metal substrate, typically an aluminum substrate. Prior to sputtering, the substrate is textured by grinding, typically using a rotary abrasive pad placed off center with respect to the surface of the spinning substrate. The purpose of the texturing is create a roughened surface characterized by submicron surface irregularities. The roughened surface reduces stiction between the disc and head by reducing surface contact between the two.
Before texturing, the metal substrate is plated with a selected alloy, such as nickel/phosphorus, to achieve a requisite surface hardness. The plated disc is then polished to remove surface nodules which form during the plating process. Because the nodules have varying degrees of hardness, the polishing step tends to leave surface irregularities in the form of surface depressions or mounds.
The sputtering operation used to produce the thin magnetic film is preferably carried out by first sputtering a chromium underlayer onto the substrate surface, then sputtering a cobalt-based magnetic thin film over the underlayer. A protective, lubricating carbon overcoat may then be applied over the thin-film layer by sputtering. The resulting disc can have high coercivity and remanence properties, as described in co-owned U.S. Pat. No. 4,816,127, and good wear and lubricity properties, as described in co-owned U.S. patent applications Ser. No. 341,550 filed Apr. 21, 1989, and Ser. No. 341,705, filed Apr. 21, 1989.
Despite the favorable magnetic and surface-wear properties which can be achieved in the above-described thin-film magnetic disc formed on a metal substrate, the recording density of the disc is limited in flying height by the irregularities on the disc surface (due to surface irregularities in the plated metal substrate). The best flying head distances which have been achieved with metal-substrate discs is about 6 .mu.inches.
It is possible to reduce flying height, and therefore increase recording density, by forming a thin-film magnetic layer on a smooth-surfaced substrate, such as a highly polished glass or ceramic substrate. However, this approach has generally been limited by difficulties which have been encountered in (a) achieving a high-coercivity, high-remanence, low bit shift magnetic film on a glass substrate by sputtering, and (b) texturing the substrate to produce a low-stiction disc surface The former limitation has been addressed in co-owned U.S. patent application Ser. No. 408,655 filed Sept. 19, 1989. The second limitation is the subject of the present application.